1. Field of the Invention
The present invention relates to a cryogenic receiving amplifier and an amplifying method thereof, both of which are used in a receiving system for a radio communication base station, and so forth.
2. Description of the Related Art
As an amplifier operable in an environment of a cryogenic temperature, non-patent literature 1 discloses a receiving amplifier for a satellite earth station, another receiving amplifier for radio astronomy, and so forth. Further, non-patent literature 2 discloses a study on application of a superconducting filter to a receiving system for a mobile communication base station. In the study, the superconducting filter and a cryogenic receiving amplifier form a cryogenic receiver front end. For cooling these devices, use is made of liquid nitrogen, liquid helium, or a vacuum container, thereby lowering the temperature of the receiver front end to an extent from ten and several Kelvins to around sixty Kelvins. In both the non-patent literatures, the receiving amplifier can reduce a noise figure under the cryogenic temperature environment, while the receiving amplifier can achieve a highly sensitive reception.
As disclosed in non-patent literature 3 , a high electron mobility transistor (hereinafter referred to as HEMT) or a field effect transistor (hereinafter referred to as FET) is employed as a microwave semiconductor which is used in a cryogenic receiving amplifier for the purpose of reducing the noise figure. As disclosed in non-patent literature 4, it is generally known that the noise figure of a HEMT is superior to that of an FET in cryogenic temperature. HEMTs are typically made of gallium arsenide (hereinafter referred to as GaAs). With a GaAs HEMT, it is possible to obtain the noise figure of around 0.3 dB, but saturation power of at most around 15 dBm. On the other hand, with a GaAs FET, though it is impossible to obtain the same low noise figure as that of the GaAs HEMT, the saturation power of around 35 dBm can be obtained.
Non-patent literature 5 discloses another study in which features of these transistors are combined in a cryogenic receiving amplifier for a mobile communication base station. Especially, the non-patent literature 5 proposes the cryogenic receiving amplifier having a three-stage amplifier configuration including a GaAs HEMT disposed at a first stage, a GaAs FET at a second stage, and a GaAs FET at a third stage. The illustrative configuration can realize a low noise figure and a high saturation power because of the combination of the low-noise-figure HEMT disposed at the first stage and the high-saturation-power FETs disposed at the second and the subsequent stages. The non-patent literature 5 shows the noise figure of 0.25 dB, a gain of 43 dB, an output intercept point of 38.5 dBm, and a maximum power added efficiency of 15% or less. In a receiving system for a mobile communication base station, it is necessary to simultaneously amplify radio waves from mobile terminals which are positioned at different distances from the base station in the same cell. For this purpose, the output intercept point is required to be from 1 to 2 W.
Referring to non-patent literature 6 , in recent years, an increasing study has been made on a gallium nitride high electron mobility transistor (hereinafter also referred to as GaN HEMT) as a high-power microwave semiconductor. A feature is that the GaN HEMT can be operated at a higher drain-source voltage than that of the GaAs FET. Thus, an amplifier can be implemented with a high load impedance, resulting in a reduction in loss in a matching circuit. There is an additional advantage in that a higher operating temperature can be set in the GaN HEMT, that is, the GaN HEMT has a high heat tolerance. It is thereby possible to provide a downsized radiator to externally dissipate the heat produced by the GaN HEMT, thus realizing a smaller and lighter amplifier as a whole. Because of these features, discussions have been made on application of a GaN HEMT to a transmitting amplifier for a base station as a high-power microwave semiconductor used at a room temperature.
Non-patent literature 1: Hamabe, Saito, Ohmura, Mimino, “Cryogenic HEMT amplifier”, IEICE Technical Report (Electron Devices), ED88-122, January 1989.
Non-patent literature 2: T. Nojima, S. Narahashi, T. Mimura, K. Satoh, Y. Suzuki, “2-GHz band cryogenic receiver front end for mobile communication base station systems”, IEICE Transactions on Communications, vol. E83-B, no. 9, pp. 1834-1843, August 2000.
Non-patent literature 3: M. W. Pospieszalski, S. Weinreb, R. D. Norrod, and R. Harris, “FET's and HEMT's at cryogenic temperatures—their properties and use in low-noise amplifiers—”, IEEE Transactions on Microwave Theory and Techniques, vol. 36, no. 3, pp. 552-560, March 1988.
Non-patent literature 4: K. H. G. Duh, M. W. Pospieszalski, W. F. Kopp, A. A. Jabra, P-C Chao, P. M. Smith, L. F. Lester, J. M. Ballingall, and S. Weinreb, “Ultra-low-noise cryogenic high-electron-mobility transistors”, IEEE Transactions on Electron Devices, vol. 35, no. 3, pp. 249-256, March 1988.
Non-patent literature 5: Mimura, Narahashi, Nojima, “A 2 GHz band cryogenic three-stage amplifier for cellular base station receivers”, 1999 IEICE General Conference, B-5-31, March 1999.
Non-patent literature 6: T. Kikkawa, and K. Joshin, “High power GaN-HEMT for wireless base station applications”, IEICE Transactions on Electron., vol. E89-C, no. 5, pp. 608-615, May 2006.
The prior-art cryogenic receiving amplifier has been used at the class “A” bias voltage to maintain both linearity and the low noise figure. Further, since employed is a transistor with such a low saturation output power as 10 to 20 dBm, it is impossible to obtain a sufficiently high power added efficiency. Additionally, the cryogenic receiving amplifier requires three or more stages of transistors to achieve a higher saturation output power of 1 W or more. On the other hand, if, for lower power consumption of the multistage cryogenic receiving amplifier, the bias voltage of the FET at the second or subsequent stage is set to the class “AB” or “B”, there is a drawback of deteriorating overall linearity of the cryogenic receiving amplifier. Since linearity and power added efficiency are in a trade-off relation to each other, it is impossible to simultaneously achieve the saturation output power of 1 W or more and the power added efficiency of 50% (theoretical maximum value at the class “A” bias) or more in the prior-art configuration of the cryogenic receiving amplifier employing an FET which is set to the class “A” bias voltage.